1. Field of the Invention
The oxidation stability of lube oil basestocks and formulated lube oils is improved by the addition of minor amounts of tetralins, alkylated tetralins or mixtures thereof, or the combination of tetralins alkylated tetralins or mixtures thereof and organic sulfides to the oil. The lube oil may be obtained from any source such as natural naphthenic or paraffinic petroleum hydrocarbon oil, tar sands or shale oil, or synthetic oils such as poly alpha olefins, synthetic ester oils, oils produced by the isomerization of slack waxes or hydrocarbon synthesis waxes (e.g. Fischer-Tropsch waxes), etc.
2. Description of the Prior Art
The inhibition or elimination of oxidation in lube oils is a long sought after goal. With increased levels of refining of lube oils to meet ever increasing performance demands and oil specifications regarding viscosity index, color, viscosity, cold cranking simulation performance, etc., natural anti-oxidants are removed from the oil and oxidation becomes a problem to be addressed because associated with oil oxidation is an increase in the tendency of an oil to form acidic components which render the oil corrosive.
U.S. Pat. No. 3,812,035 teaches a lubricating oil composition which contains A(i) 5-30 wt % trialkyl-substituted tetrahydronaphthalene (trialkylsubstituted tetralin), (ii) and 61-92 wt % di-n-(C.sub.6 -C.sub.18) alkyl-phenyl hydrocarbons and B 11-100 pts/100 part A of a mineral lube oil selected from pale oil, naphthenic oil and bright stock. The resulting oil combination is reported as exhibiting a synergistic effect with respect to oxidation resistance and solidification point.
U.S. Pat. No. 2,427,766 teaches a compounded lubricating oil which exhibits resistance to oxidation. The oils are highly refined and substantially free of olefinic hydrocarbons and contain two stabilizing additives selected form the classes of oil-soluble organic sulfides and polynuclear aromatic compounds. The patent reports that the aromatic hydrocarbons have little anti-oxidant properties by themselves.
GB 1,296,907 teaches an anti-oxidant composition. That composition comprises 5-70 parts by weight 9, 10 di hydroanthracene which exhibits synergistic interaction when combined with 2 to 30 parts by weight of certain known anti-oxidants. The 9, 10-di hydroanthracene is described as being in itself oxidatively unstable, but synergistically anti-oxidative when combined with peroxide decomposing anti-oxidants such as dialkyl and diaryl monosulfides, di alkyl and diaryl disulfides, zinc dialkyl dithiophosphates and dialkyl thiocarboxylates. This combination is useful for stabilizing mineral oils and synthetic esters. The patent also indicates, however, that there was little or no anti-oxidation synergism for the combination of 2-n-propyltetralin and organic sulfide when used in liquid paraffin oils.
Japanese 01-152188 discloses a process wherein fluid cat cracked fuel oil is color stabilized by hydrogenation. The hydrogenation process produces greater than 0.5% tetralin or indane in the fuel oil. The addition of C.sub.4 -C.sub.15 alkyl sulfides to the fuel oil further stabilizes color. Alternatively, the hydrogenated oil can be mixed with 0.2 to 5 wt % of a lube oil containing 200 ppm or more (based on sulfur) of a sulfide and 5 wt % or more tetralin and/or indane producing a fuel oil of good color stability.